Millsite

Chemical formula: Cu<sup>2+</sup>Te<sup>4+</sup>O<sub>3</sub>·2H<sub>2</sub>O

Millsite is a very rare copper tellurite, forming microscopic, platy crystals of an intense blue color.

## Characteristics Millsite is a hydrated copper tellurite, occurring as extremely small, thin, platy crystals that rarely exceed 0.2 mm. These crystals form rosette-like or spherical aggregates, as well as coatings and crusts. Due to their microscopic size, visual features are difficult to observe without magnification. ## Physical Properties Millsite crystals exhibit a vitreous luster. Hardness and density have not been precisely measured due to the small size and rarity of samples. It is a brittle mineral. ## Colors and Varieties Millsite is characterized by an intense, azure-blue color. It has no known varieties. ## History and Name The mineral was named after Stuart J. Mills (born 1979), an Australian mineralogist and crystallographer, for his contributions to the study of tellurium minerals. It was approved by the IMA in 2012 (IMA2012-064), and its description was published in 2013. The type locality is the mines in the Tombstone area, Arizona, USA. ## Uses Due to its extreme rarity and microscopic size, millsite has no industrial applications and is solely an object of scientific interest and for specialized micromineral collectors.

Properties

Mohs hardness
2
Luster
Vitreous
Streak
Light blue
Density
3.63
Cleavage
Perfect on {010}
Fracture
Brittle
Transparency
Transparent
Crystal system
Orthorhombic

Diagnostic features

## Identification Identification of millsite is based on its characteristic, intense blue color, the form of microscopic, platy crystals forming rosette-like aggregates, and its association with other rare tellurites and tellurides. Identification requires specialized equipment, such as a stereoscopic microscope, and final confirmation is only possible using advanced analytical methods (e.g., EDS, XRD). ## Distinguishing from Similar Minerals Millsite can be confused with other blue secondary minerals, such as linarite, azurite, or chalcanthite. However, it is distinguished by its mineral association (occurrence with tellurium minerals), specific crystal form (thin plates in rosettes), and geological environment. Many of these minerals also have a different reaction to water or acids. ## Crystal Forms Millsite forms very thin, platy crystals with a hexagonal or rhombic outline. These crystals often combine into rosettes, spherical aggregates, or form thin coatings on the surface of the host rock.

Geological environment

## Genesis Millsite is a secondary mineral, forming in the oxidation zones (gossans) of hydrothermal ore deposits rich in tellurium. It forms under low-temperature conditions as a result of the weathering of primary tellurides. ## Mineral Associations This mineral co-occurs with other, often rare, tellurium minerals. At the type locality (Tombstone, Arizona), it has been found in association with quartz, rodalquilarite, tellurite, paratellurite, eztlantite, oboyerite, jensenite, and frankhawthorneite. ## Localities The most important and well-documented occurrences of millsite worldwide are the mines in the Tombstone area (Cochise County, Arizona, USA), which is its type locality. It has also been identified in the El Plomo mine in the Atacama Department, Chile.

Rarity

Extremely rare

For collectors

## Quality Criteria For micromineral collectors, the most valuable specimens are those with clearly formed, undamaged, rosette-like crystal aggregates of an intense blue color. Contrast with the rock matrix and the richness of association with other rare tellurium minerals are also important. The size of the aggregates, even if measured in fractions of a millimeter, is significant. ## Popular Localities The only source of collectible specimens are the mines in the Tombstone area, Arizona (USA), which is the mineral's type locality. Specimens from this locality are the most prized.

Care and storage

## Cleaning Specimens with millsite should be cleaned only very carefully, using compressed air to remove dust. Any contact with water or chemicals is highly inadvisable. Mechanical cleaning (brushes, needles) is impossible and will destroy the microscopic crystals. ## What to Avoid Absolutely avoid contact with water, acids, detergents, and all other liquids. The mineral is likely sensitive to changes in temperature and humidity. Do not heat it or expose it to direct sunlight. ## Storage Specimens should be stored under stable conditions, in closed, airtight containers (such as "micromount boxes"), to protect them from dust, humidity, and mechanical damage. It is best to keep them in a dry place with a constant temperature.

Sources

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